The invention is directed to a method for the compensation of geometric image errors in video images with a plurality of lines, each line having a plurality of image points, wherein the image errors to be compensated influence the length of the lines and the ith line in geometric image errors that have not been compensated begins at a location xai and ends at a location xei on a projection surface. The invention is further directed to an arrangement for displaying video images on a projection surface in which image points in a plurality of lines are sequentially illuminated and geometric image errors are compensated in accordance with the method, wherein these image errors to be compensated influence the length of the lines and the ith line in geometric image errors that have not been compensated begins at a location xai and ends at a location xei on a projection surface. Moreover, the invention is directed to an arrangement for displaying video images on a projection surface in which the display is carried out on this projection surface at an inclination.
Geometric image errors of the type mentioned above can occur, for example, when a transparency is projected at an angle. For an overhead projector, a mirror is provided for compensation according to WO 97/03380, wherein the image is projected onto the back of a projection surface by means of the mirror. The inclination of the mirror relative to the projection surface is arranged in such a way for compensating trapezoid distortions that the light paths from the projector to the projection surface are approximately identical in all areas of the image.
In a video projection according to DE 32 43 879 C2, mirrors are also used, as in the above-mentioned projection with the overhead projector, to compensate for the trapezoid distortions occurring through diagonal projection.
The above-mentioned compensation with mirrors requires very large mirrors for large-image projection. Therefore, it would take up much space and is substantially limited to rear projection so that the mirrors do not conceal the projection surface from view.
In the video technique disclosed in EP 0 756 425 A2, a liquid crystal matrix controlled with a video image is projected onto a screen. In this case, trapezoid distortions are compensated without mirrors in that all lines of the image are shortened with reference to the length of the shortest line. For this purpose, the picture is distorted on the liquid crystal matrix in a trapezoidal shape in such a way as to compensate precisely for this distortion due to diagonal projection. This type of distortion of the projected image is carried out in the case of LCD images in that image points are omitted in the shortened lines. At very large angles, however, it is to be expected that the loss of resolution will be so great that it will no longer be possible to display a high-quality pictures.
In diagonal projection of video images, trapezoid distortions of the type mentioned above change the line length, for example, when the orientation of the frame scanning is carried out at a small angle. This also changes the related density of image points in the line, so that these errors can essentially only be compensated through correction of the line information.
It is the object of the invention to provide a method for compensation of image errors of the type mentioned above and an arrangement for carrying out this method in which, in the case of large angles, loss of information due to missing image points is reduced.
This object is met through a method of the type mentioned above in which a substantially parallel light bundle is deflected on the projection surface in two dimensions for sequential illumination of the image points of the video image, a light bundle for the image point at every location on which the light bundle is deflected is intensity-modulated at this location in accordance with the image point information of the undistorted video image, a value determining the start of the line of the compensated image is defined by xadxe2x89xa7Max(xai) and a value determining the end of the line of the compensated image is defined by xedxe2x89xa6Min(xei), where xed greater than xad, and the light bundle for each line is deflected in such a way that all image points of the line i are displayed sequentially within the area [xad; xed] on the projection surface. An arrangement according to the invention for carrying out the method comprises a source for the emission of a substantially parallel light bundle for a sequential illumination of image points of the video image, which source can be intensity-modulated, a deflection device for scanning the light bundle in two dimensions, a storage for the sequential storage of line information for the intensity modulation of the source for N image points, two values xad and xed, where xed greater than xad, wherein xad describes the start of the line of the compensated image, where xadxe2x89xa7Max(xai) of all lines i, and xed describes the end of the line of the compensated image, where xadxe2x89xa7Min(xei), and a control device for modulating the source and for controlling the deflection device in accordance with functions by which the light bundle is deflected and/or intensity-modulated in such a way that all image points of the line i for deflection can be displayed on the projection surface sequentially within the area [xad; xed]. Further, in accordance with the invention, there are provided in a special arrangement for the compensation of errors in diagonal projection a source for the emission of a substantially parallel light bundle for a sequential illumination of image points of the video image, which source can be intensity-modulated, a deflection device for scanning the light bundle in two dimensions, and a control device which controls the intensity modulation for the light bundle as well as the deflection of the light bundle in accordance with a function that is obtained through a calculated distortion correction of the image, at least with respect to the inclination.
Therefore, the technique employed in accordance with the invention is totally different from that used in EP 0 756 425 A2. That is, instead of a LCD matrix, substantially parallel light bundles which can be generated, for example, by a laser are used for sequential scanning of an image. Accordingly, there is no dependence on the matrix of an image. This technique has the advantage that the image is always sharp regardless of distance and even without special optics. The sharpness is limited practically only by the diameter of the light bundle. Therefore, by suitable control, an image can be distorted, also without loss of resolution, in such a way that a distortion expressing itself by a change in line length, for example, is compensated.
In particular, in the arrangement for displaying an image by diagonal projection, the image contents are calculated anew and the line density as well as the image point density in the recalculated image are displayed in a distorted manner such that the distortion causes, through the projection, a distortion correction of the video image. In this way, video images can be displayed practically as precisely as desired. Naturally, the accuracy of the calculation and display is substantially higher for CAD applications than for a television picture because the resolving capacity of the human eye is not as high. However, the principle is the same. Subsequent embodiment examples will provide more detailed information on the calculation of the distorted images which are then displayed in a rectified or distortion-corrected manner.
This calculation need not necessarily be carried out anew for every video image. For example, it is sufficient that the video images which are distorted for the purpose of distortion correction are stored on a videotape and then simply read out from the videotape in subsequent displaying of the video image. Further, the calculations are also not limited only to diagonal projection. Other optical influences such as the deflection behavior of the deflection device can also be taken into account in the recalculation. The subsequent embodiment examples will also provide further details on this matter.
In a preferable further development of the invention, it is provided in the method that line information determined for the intensity modulation of the light bundle is stored sequentially in a storage as N image points and the light bundle is blanked at the start of each line i during a time interval for scanning the length (xadxe2x88x92xai), whereupon the information determined for intensity modulation for the N image points is read out of the storage within a time interval for scanning length (xedxe2x88x92xad) and the light bundle is intensity-modulated within this time interval with respect to this read-out information, and at the conclusion of the sequential illumination of the N image points on the projection surface the light bundle is blanked for the remainder of the time interval T for the scanning of each line. In a preferable further development of the invention, a fixed time interval T is predetermined for all lines and the control device is intensity-modulated according to a function by means of which the light bundle is blanked at the beginning of each start of a line i during a time interval for scanning the length (xadxe2x88x92xai), whereupon the information for the N image points which is determined for intensity modulation is read out of the storage within a time interval for scanning the length (xedxe2x88x92xad), and the source is intensity-modulated with this information, and at the conclusion of the sequential illumination of the N image points on the projection surface the light bundle is blanked for the remainder of the time interval T for scanning each line.
Accordingly, a time control is provided for controlling compensation instead of a control with different line deflection functions for scanning different lines, which would also be possible. For this purpose, an inner area in which the image is displayed is selected for the compensation process from the locations on the projection surface that are accessible through line deflection and frame deflection, that is, by means of the extrema xai and xei given by the line scanning of line i. This type of selection leads to the determination of two values xad and xed which describe the starting location and the ending location of the lines of the displayed image on the projection surface. The position of these values will be illustrated more clearly hereinafter with reference to the Figures.
The time control provided works in such a way that every line of the video image is scanned within the same time interval T, wherein the areas occurring outside of the area designated by xad and xed as a result of the trapezoid error are blanked, while subsequently, for the N image points in the region in which the light bundle is located on the projection surface in the interval between xed and xad, the stored line information in its entirety is written onto the projection surface in a geometrically correct manner. If the projection angles are not too large, that is, when only slight distortions are to be compensated, the image points can be displayed in a time interval (xedxe2x88x92xad)*T/[N*(xeixe2x88x92xai)] that is identical for all image points. However, in the case of high nonlinear distortion of line information, it is recommended that the time intervals for displaying the successive individual image points are suitably selected in accordance with the distortion to be compensated. The required compensation can then be calculated for different arrangements in a manner familiar to those skilled in the art of optics by means of geometrical optics in that the projection surface is occupied by image points and the light paths for light bundles which illuminate these image points are traced back to the deflection device where the relationship between the deflection angle and the scanning time for imaging every image point is compulsorily given.
In another preferable further development of the invention, the intensity of the light bundle for every image point is controlled in inverse proportion to its illumination time. According to this further development, the intensities are corrected in continuously operating sources corresponding to different scanning times and, therefore, corresponding to different illumination times. For this purpose, an attenuator can be provided for the arrangement, in particular. Attenuation is to be achieved rather than amplification because it can be ensured in this way that the output limits of the lasers, mentioned by way of example, for generating the light bundles are not exceeded.
This controlling with reference to the correct image point intensity can be carried out at different points in the control. According to a preferred further development, the proportional controlling is carried out after the readout of information from the storage. In this way, information length in the storage can be economized on because, otherwise, the necessary increased dynamic of the information due to the scaling with respect to the illumination time would increase the required word length of the storage brought about by the image resolution.
Surprisingly, it has been shown that, according to the invention, a distortion can be carried out in the direction of the frame deflection, that is, vertical to the lines. For this purpose, scanning over a constant change in deflection angle which is common in television technology can be dispensed with and the controlling may be selected according to a function in which the line spacing in the projected image is again uniform. In this regard, an advantageous further development of the invention provides that the light bundle is scanned framewise with a function on the basis of which the line spacings of adjacent lines in the total image differ from one another by a maximum of 30% and, in particular, by less than 10%. In a corresponding arrangement, the deflection device for framewise deflection is controlled diverging from a constant change in the deflection angle with a function on the basis of which the line spacings of adjacent lines in the total image differ from one another by a maximum of 30% and especially by less than 10%. The indicated limits of 10% and 30% are sufficient to prevent possibly remaining line spacings from being perceived at a suitable distance. On the other hand, the indicated tolerances also allow an image deflection by means of frame mirrors which, because of the mechanical movement and the resulting inertia, can not exactly follow the predetermined function for compensation of line spacing unconditionally for all lines of the deflection.
The function for controlling is determined geometrically, for example, in such a way that the area of the projection surface available for the video image is uniformly occupied by image points, and the light bundles made possible by the arrangement for the illumination of image points are traced back to the deflection device in order to determine the relationship between the deflection angle and line. In extreme cases, when projecting on a curved projection surface or when the image is even rotated during projection, the deflection can also depend on the position of the respective image point of the line, so that at least a linear component of the line deflection can be mixed into the frame deflection, and/or vice versa, and the image information is then also no longer read out of the image storage in two dimensions, but rather its addresses for the readout of information can also be suitably formed from the input addresses in this case. Other methods for this purpose are described more fully hereinafter with reference to the embodiment examples.
Similarly, the limits for the image point spacings within a line that are still within tolerances can also be specified. In the case of a continuously writing light bundle, however, there are no image points within the actual meaning of the term; for this reason, these limits for the distorted deflection are to be compared by way of the displayed video information Vi(x) with the video information ViT(x) which would result if a completely undistorted image were displayed. Accordingly, it is provided in a further development that the light bundle is scanned linewise by a function in which the video information Vi(x) of the line i for the image information at every location x differs with respect to video information ViT(x) of an undistorted image by a maximum amount             "LeftBracketingBar"                                    V                          i              ⁢                              xe2x80x83                            ⁢              T                                ⁢                      (            x            )                          -                              V            i                    ⁢                      (            x            )                              "RightBracketingBar"        =          "LeftBracketingBar"                                                  ∂              V                        ⁢                          xe2x80x83                        ⁢            i                                ∂            x                          ⁢                  Δx          i                    "RightBracketingBar"        ,
where the value xcex94xi determined by this equation is less than 0.3-times the line length, especially less than 0.1-times the line length, divided by the number of image points of the video image according to the video standard. In this respect, it is provided for an arrangement that the deflection device with respect to line deflection is controlled in divergence from a constant change in deflection angle at which the video information Vi(x) of the line i for the image information at every location x differs with respect to video information ViT(x) of an undistorted image by a maximum amount             "LeftBracketingBar"                                    V                          i              ⁢                              xe2x80x83                            ⁢              T                                ⁢                      (            x            )                          -                              V            i                    ⁢                      (            x            )                              "RightBracketingBar"        =          "LeftBracketingBar"                                                  ∂              V                        ⁢                          xe2x80x83                        ⁢            i                                ∂            x                          ⁢                  Δx          i                    "RightBracketingBar"        ,
where the value xcex94xi determined by this equation is less than 0.3-times the line length, especially less than 0.1-times the line length, divided by the number of image points according to the video standard.
As has already been made clear, the image point density can depend heavily upon the line and the position of the respective image point in the line. In the same way, the light spot of the substantially parallel light bundle on the projection surface is also larger or smaller depending on the image point to be illuminated. For this reason, the diameter of the light bundle is generally selected such that, even in the least favorable image point locations with respect to the achievable resolution, suitably large image points can always be displayed. However, for other image areas within the image this means that the resolution could be increased. Interpolation algorithms of a known type can be used for this increase in resolution in order to generate the additional image points. Information is gained in this way without increasing the number of lines in general because the image point density in video images is always limited by the transmission bandwidth for the image. Therefore, it is generally sufficient for generating additional image points to provide a larger quantity N of storage locations for the interpolation of the information of a line and to carry out the scanning of the analog video signal for storage at a higher frequency than that prescribed based on the image point frequency for displaying lines in the video standard. Therefore, according to a preferable further development of the invention, it is provided that the number N is greater than the quantity of image points of the video standard of the video image to be displayed.
The analog video signal is accordingly already scanned before storage with a correspondingly higher resolution and is then available for displaying in smaller time intervals also with increased resolution.
The increase in the quantity N has still further advantages in another further development of the invention. This further development of the invention is characterized in that the control device also stores in the storage the information for dark image points which is required before and after the time interval for scanning the length (xedxe2x88x92xad), and the line information in the storage generated in this way can be supplied in its entirety to the deflection device during time T. The deflection device can always be operated in the same manner for the readout of the storage. The preparation of the data for the image points that are blanked according to the invention by storing corresponding information in the storage generates in a storage line the entire line to be scanned. This has substantial advantages with respect to the circuitry required for the correction of geometric image errors, as will also be made clearer hereinafter with the aid of embodiment examples. In particular, it is also possible in a simpler manner in this further development to carry out image transformation with respect to the geometric image errors in two directions in real time, which would only be possible with especially fast electronics with distortions in two directions because of high video frequencies.
As has already been mentioned, it is extremely advantageous according to an advantageous further development of the invention that before displaying the video image the image is calculated anew with respect to the deflections and spatial correlation of the image points for displaying an undistorted image.
The possibility for compensation, according to the invention, of geometric image errors of the type mentioned above provides an unexpected advantage. As a result of the compensation method, a laser video device can be arranged, as in the prior art, at a greater angle to the projection surface when projecting a video image on a wall of a room, for example, on the ceiling.
The image distortion caused by projecting at an angle can likewise be compensated with the method. Given a suitable arrangement of the projection surface and laser video system, it is even possible to select a projection geometry that rules out the possibility of a person entering the area of laser light, so that applicable legal requirements for laser safety can be met more easily or even without additional expenditure. In this regard, especially, a preferred further development of the arrangement according to the invention provides the following: A first component group which comprises the deflection device and at least one socket for inserting a light-conducting fiber and within which the light introduced into the socket is guided for deflection into the deflection device, a component group which is separate from the first component group and which has the control device and the source which can be intensity-modulated and at least one socket for insertion of a light-conducting fiber and within which the light of the intensity-modulated source is conducted into this socket, at least one light-conducting fiber for coupling the first component group with the second component group via the respective sockets and a fastening device for the first component group by which the first component group can be arranged at an angle to the projection surface, wherein the compensation is configured for correcting the distortion given by imaging at this angle.
Due to the fact that the deflection device is separated from the component group containing the lasers and their modulation control, it is necessary to fasten only a small, light projection head as first component group to the ceiling of the room, as mentioned by way of example, which can easily be accomplished by the average user with little mechanical know-how. In this respect, it is noted that this should be just as simple as hanging a lamp, which the average consumer can carry out independently without employing an electrician. The essential electronic devices, both the laser and modulators, are arranged in a second component group which can also contain the operating controls. The second component group can be arranged, for example, on the floor or on a shelf.
The transmission of image information between the two component groups is carried out by light-conducting fibers. For this purpose, a socket is provided in the separate component groups for insertion of the light-conducting fibers. This socket likewise facilitates the installation of a video system which is divided into two component groups in the manner mentioned above. Further, the second component group formed of lasers, modulators and other control devices can also be disconnected easily because of this construction when sent out for maintenance and repair.
As was already stated above, it is particularly advantageous when fastening means are provided for the first component group for fastening the projection head at the ceiling, the wall or floor of a room, and fastening means are provided for fastening a screen serving as projection surface to the wall. In the case of movable video devices, for example, video projection devices which are to be used in various rooms in a conference center, the first component group exclusively can be fastened in every room to the ceiling and, because of the simple connection by light-conducting fibers, the second component group can be connected only in the room in which the video projection device is to be used. In this case, a minimum number of laser devices will be sufficient for different conference purposes in the conference center, which sharply reduces costs for outfitting with laser projectors of this type.
In another advantageous further development, especially when small lasers such as solid-state lasers are used, it is provided that the first component group and the second component group are combined in a housing and this housing has fastening means for fastening to a wall, ceiling or floor of a room.
However, in another advantageous further development of the invention which is particularly suitable for conference centers of the type mentioned above, a holding device is provided at the edge, especially the upper edge, of the projection surface at which the first component group is fastened so as to be off-center with respect to the projection surface, so that the video image is displayed at an angle.
In this case, the first component group is fixedly connected with the projection surface and the entire projection surface with the firste component group and, as the case may be, also the second component group, can be moved from one room to another. This likewise economizes on the cost of a plurality of first component groups, one for each room.
In this further development, the entire video system and especially the projection head is also always precisely aligned with respect to a screen, mentioned by way of example, serving as projection surface, so that there is no need for adjustment work for operating in different rooms. In this case, the entire video system should also be mounted on rollers to facilitate transport.
A video projection system including, in particular, the invention and its further developments substantially comprises an electronic control unit, an input module, a control circuit for image point scanning and line scanning, and a device for image calculation. Further, the images should be written continuously, so that a brightness-modulated and color-modulated collinear light source should be provided which couples light into a deflection system, wherein the deflection system should be biaxial for displaying video images. In particular, advantages are provided with respect to angular magnification when magnification optics are provided, as will be seen from the following embodiment examples. In this case, the biaxial deflection system can comprise, for example, a nutating mirror or a line mirror and a frame mirror or can comprise one or more nonmechanical deflectors or a combination of different deflectors. In particular, the magnification optics should be corrected in accordance with the tangent condition so as to be free of distortion, and the origin of the beam deflection should lie, actually or apparently, in the deflection system, so that the deflection is carried out from one spatial point in practice. The projection surface required for displaying the image can be arranged for rear projection or front projection.
For the purpose of carrying out the method according to the invention in a particularly advantageous manner, the control circuit for image point scanning and line scanning should calculate a geometry-optimized image point scanning function both in the line direction and frame direction depending on the parameters of the deflection system, on a diagonal position in two angular directions, and on the surface shape of the projection surface. Thus, a new calculation of the image is carried out in particular in the unit for image calculation by these optimized scanning functions based on the incoming video data.
The invention and the further development involve both method features and device features for the correction of line spacings. Accordingly, on the one hand, the tangent error in the frame direction and different line spacings with diagonal projection can be corrected. On the other hand, a scaling of the line length is to be carried out. This also includes corrections for pincushion distortion for a biaxial scanning arrangement and a correction of the line length resulting from diagonal projection. It is further provided that the image point spacings within each line are corrected in such a way that the image distortion due to the tangent error in the line direction and a diagonal projection can be corrected through variably adjustable image point spacings between two adjacent image points. In addition, the correlation of the video information relative to the scanned pixels of an image can be determined in such a way that an image which is extensively free from distortion is obtained by means of a recalculation of the image taking into account the distorting effect of the projection surface and the distorting effect resulting from the position of the projector relative to the projection surface. Therefore, as will be clear especially from subsequent embodiment examples, a diagonal projection can also be corrected in the line direction in a simple manner. This diagonal projection in the line direction is reduced to a diagonal projection in the frame direction.
In order to achieve optimum image quality, a correction of the line spacings, line lengths, and image point spacings is carried out initially, followed by a complete recalculation of the image points corresponding to the corrected scanning geometry. Based on the complex of possibilities mentioned herein, a very high image quality is achieved with respect to faithful reproduction of detail and color purity. This is also highly desirable especially when arrangements of this type are used for CAD or printing technology. On the other hand, it is also possible to deliberately distort a given image content by means of the methods indicated herein in order to achieve intentional effects for image display. This can be advantageous particularly for advertising and entertainment applications in order to achieve special optical effects to attract the attention of the viewer.
The image distortion correction and image distortion according to the invention is made possible through comparatively simple techniques. Some of the corrections indicated hereinafter are carried out exclusively by means of computer programming steps in electronic units which are, in part, already provided in known projection systems. The extra expenditure on additional units is negligible. However, it is not necessary to manipulate the optical channel, which is a decisive advantage compared with known systems in which, for example, in order to prevent loss of image points, the image point raster of the LCD matrix could be selected in a suitably distorted manner.
It is possible to achieve a wide variety of display effects by the methods and arrangement shown herein. These display effects will generally consist in reproducing the original image contents on the projection surface with as little distortion as possible.
By means of the methods presented in this context, an image can be pre-distorted within wide limits corresponding to the distorting effect of the image generating system, the position and direction of the image generating system relative to the projection surface, and the distorting effect of the projection system relative to the projection surface of the screen. In this case, an image which is free from distortion to a very great extent can be displayed, for example, even on an irregularly shaped projection screen. The basis for this consists in that the distorting effect of projection on the projection surface can be determined and that the projection system is capable of purposely changing the image composed of image points in lines by means of the results so determined.
A first excellent advantage of a projection system working with collinear laser beams that are deflected in an angle-proportional manner consists in that the image sharpness is not dependent on the projection distance. In this case, the image size increases in proportion to the distance between the projection head and the screen. This eliminates a disadvantage of known image projectors in which a sharp image can only be displayed in a limited depth area.
A further advantage of a video projector system which works with scanned collinear light beams is that there is no need for a fixed predetermined relationship between the video data at the input of the projection system and the RGB image data at the electronic output of the system.
This applies to the time sequence as well as to the spatial allocation of the image information. Accordingly, it is possible, when the distortion characteristics of the image generating system and projection surface are known, to take these distortion characteristics into account with the incoming video data stream in real time and to read out pre-distorted RGB image data.
The RGB light beam bundles are modulated with respect to brightness, color and direction in such a way that a very extensively sharp, undistorted image or a sharp, deliberately distorted image can be displayed on a projection surface having virtually any shape. Limits are imposed only by the degree of inclination or curvature of the projection screen, since reflection conditions and scattering conditions prohibiting a high-quality image display on conventional projection screens occur when the angle between the incident light beams and the normal to the projection surface exceeds approximately 45xc2x0.
However, known projection screens exist with which larger projection angles can be used for generating images (screen according to U.S. Pat. No. 4,003,080).
It is further possible, in the case of fixed predetermined projection ratios, to store video information on a data medium so as to be pre-processed already with the information containing the desired distortion, so that a real-time processing of the incoming video data supplied to the projector is not required, enabling a considerable reduction in the cost of the projection system.
However, in a further development of the invention, the information for the correction of the line length within an image and the correction of the image point spacings within a line is recorded in the storage medium and transmitted directly to the projection system after readout of the video information from the storage medium. The storage medium can be a videotape, for example.
In this case, the video projection system requires only one circuit which obtains from the video data stream the control signal for the line spacing, with respect to the example for deflection of the frame mirror, and a control signal for the image point spacing, with respect to the example for modulation of the image point frequency. Since this construction always relates to specific applications, no problems are caused by the need to impress additional control signals on video information that is standardized per se.
It is further provided that a subassembly of the video projection system, namely the projection head, can be moved within the room while the image is being projected. This movement information can also be contained on a storage medium. It is accordingly ensured that the image display is corrected for the precise position occupied by the projector in relation to the projection surface.
The invention will be described more fully in principle hereinafter with reference to embodiment examples in connection with the drawings.